In processes wherein silicon fluoride compounds are burned or pyrolyzed in the flame of a combustible mixture of fuel and oxygen, hydrophobic SiO.sub.2 is formed which is ultrafine and which is normally difficult, if not impossible, to efficiently separate from the hot, gaseous product stream in conventional methods, such as cyclones or filters. This ultra-fine silica is known as "fumed silica". Some of the SiO.sub.2 may be separated by such conventional methods but uneconomical amounts of the ultra-fine SiO.sub.2 pass through the collection devices. Some degree of recovery of the SiO.sub.2 is possible using electrostatic precipitators, but the product which is recovered by the electrostatic precipitators is often of irregular particle size due to inconsistent agglomeration on the collection surfaces of the precipitators and due to the fact that some classification can occur by way of having some particles held up briefly while others go through virtually unchecked. Often some of the SiO.sub.2 is recovered as a coarse material with a particle size approaching that of sand and thus has no appreciable value, if any, as a marketable product.
The production and collection of fumed hydrophobic silica formed by flame hydrolysis of silicon fluoride compounds should not be equated with the formation of fumed, hydrophilic silica formed by flame hydrolysis of silicon chloride compounds; such hydrophilic silica does not present the same agglomeration problems as hydrophobic silica.
It is well recognized that fumed SiO.sub.2 has appreciable market value, but SiO.sub.2 of coarse or grainy particle size approaching that of sand has little or no market value.
Representative patents for the formation of fumed hydrophobic SiO.sub.2 are, for example, U.S. Pat. Nos. 2,819,151; 3,203,759; 3,110,562; 3,645,684; 3,233,969; 3,284,167; and others. The difficulty of efficiently collecting the fumed SiO.sub.2 without appreciable loss of the SiO.sub.2 by way of chemical reaction with other components of the gas stream or by over-agglomeration or inconsistent agglomeration are problems which confront persons attempting to manufacture fumed hydrophobic SiO.sub.2.
In our U.S. Pat. No. 4,008,130, there is taught a method for preparing a ternary azeotrope containing about 36% H.sub.2 SiF.sub.6, about 10% HF, and about 54% H.sub.2 O. It is also taught that the azeotrope may be burned in a combustion chamber to produce fumed hydrophobic SiO.sub.2. When the ternary azeotrope, which is predominantly water, is burned to produce SiO.sub.2, the concentration of SiO.sub.2 carried in the hot gases leaving the burner is quite low, generally at a concentration of less than about 1.5%. It is difficult to recover a high percent of the SiO.sub.2 when the concentration of SiO.sub.2 in the gases is so low.
As compared to prior processes where SiF.sub.4 is fed to the burner to produce fumed SiO.sub.2, the use of an azeotrope feed, as in the present invention, results in a product stream which is relatively low in SiO.sub.2 content and the difficulty of recovering an appreciable percentage of the SiO.sub.2 in agglomerated, non-fused form is greatly magnified.
There exists, then, a need for methods of recovering fumed hydrophobic SiO.sub.2 which will agglomerate such low-loadings of ultra-fine silica, consistently and on a continuous basis, to an extent which renders it substantially collectable in cyclones or other dispersoid-collecting devices, but which does not cause substantially unequal residence times of some of the particles prior to being removed from the product stream. Unequal residence times can cause some of the high surface area fumed silica to fuse into relatively low surface area grainy particles.
It is an object of the present invention to provide a method for separating substantial amounts of fumed hydrophobic silica from the hot gaseous product stream leaving the combustion chamber of a burner in which an azeotrope of H.sub.2 SiF.sub.6, HF, and H.sub.2 O is burned to produce ultra-fine hydrophobic SiO.sub.2.
It is a further object to provide a method for continuously separating fumed, hydrophobic SiO.sub.2, present as a low-concentration dispersoid in a hot gaseous stream, by agglomerating the SiO.sub.2 to a particle size which is substantially constant but which is still characterized as a fumed, hydrophobic, high-surface area SiO.sub.2.
These and further objects will become apparent to practitioners of the relevant arts upon reading this disclosure.